1. Field of the Invention
The present invention relates to an autostereoscopic multi-user display having a sweet-spot unit for focussing light to form bundles of rays directed towards the eyes of one or multiple observers, and an image display matrix for alternate modulation of the light of the sweet-spot unit with two-dimensional images with monoscopic or stereoscopic image sequences. In this document, the term “multi-user display” designates a device which can be used by multiple observers for viewing alternate images of image sequences in the form of 3D scenes simultaneously and independently. The present invention takes advantage of temporal interleaving of single images for each of the observers' eyes. When employing this representation method, the resolution is maintained and is not reduced by a factor that represents the number of perspectives, as would be the case with spatial interleaving methods. The resolution is the same as the resolution of the image display matrix.
The auto stereoscopic multi-user display should meet the following conditions: ability to support multiple observers, possibility of free and independent movement of the individual observers in a wide viewing space, and selective access to several image sequences in a 2D and 3D mode. Further desired features are: high image quality, robustness, reliability, little depth, support of practically all common image and video formats, including camera and stereo-camera, and low costs.
Generally, with stereoscopic displays observers can only perceive a stereo image without cross-talking, if their eyes are located at predetermined positions. These positions are also known in the literature as sweet-spots. The present solution aims at focusing a homogeneous, large-area light distribution on extended sweet-spots, so that these sweet-spots can be tracked according to the movements of the observers' eyes and according to the currently represented right or left image using a tracking and image controller.
2. Prior Art
EP 0 881 844 A2 discloses a single-user display with light source tracking. This display works in a sequential mode. Two adjacent segments of a light source pair are provided for the left and right eye of an observer. If a segment for a right eye is turned on a first lenticular projects this segment in a multitude of images onto a diffuser plate. This plate now acts as a secondary light source, which thanks to its diffusion property permeates an image display matrix with the right image through all lenses of the second lenticular, whereby the light is focused on the right eye of an observer.
For the subsequent image, the light source pair is switched to the segment for the left eye and the image matrix is switched to the left image. If the observer moves out of the stereo zone, the second light source pair is turned on, which corresponds with the current zone.
However, it is disadvantageous to use a diffuser plate as it generally prevents multi-user operation, since the diffuser plate with its multitude of secondary light sources is represented in periodical continuation through the second lenticular.
WO 03/019952 A1 discloses a solution for multiple observers with 2D and 3D representation and a tracking system. On an image display matrix, seen from the observer, the display accommodates a controlled directing unit with two lens arrays which include a shutter, in order to focus each pixel of the image separately on one or multiple observers' eyes. Each lens array comprises a separate lens element for each pixel of the image matrix, said lens element focusing the modulated light of the pixel on the shutter. The shutter has a multitude of minute segment openings per pixel, so to open one segment per lens element for each observer, according to his eye position. The segments are projected onto the observers' eyes through a corresponding second lens element of the second lens array disposed behind the shutter. If an observer moves, a position detector transmits the observer's new position, so to only open the shutter segments which correspond with this position, in order to ensure the pixels remain focused on the eyes. During stereoscopic operation using the time-multiplex method, a right and left image are directed to the respective eyes, one after another. If there are multiple observers, multiple segments are activated. During monoscopic operation, all shutter segments may be opened.
A practical realisation of such a multi-user display is rather difficult, because a high-resolution shutter is needed. For example, one embodiment of said document having 100 possible angular positions of observers requires a segment width of 2.5 μm, assuming a typical pixel pitch of 0.25 mm. Assuming that there are 1,600 columns, a shutter with 160,000 segments per line would be required. Further, matching lens elements, shutter openings and pixels of the image matrix requires extreme precision when manufacturing the components and when aligning them during assembly. Already little deviations in pixel or segment dimensions or slight misalignment may lead to functional faults. Further, great resistance of the assembled display to ambient influences, such as temperature fluctuation or vibration, would be required.
Another major disadvantage is represented by the fact that display components cannot be replaced individually. Image display matrix, lens arrays and shutter must always precisely harmonise as regards their geometry, resolution and manufacturing tolerances.
WO 03/053072 A1 discloses a multi-user display with a 3D tracking system and sequential representation of stereoscopic images. The device comprises one behind another, a three-dimensionally addressable backlight, a single large-area, voluminous imaging lens for focusing, and a spatial light modulator as an image display matrix. The backlight allows tracking also as regards the distance between observers and the display. The focused light passes a image display matrix, which alternately modulates a left image and a right image for the respective eyes of the observers.
The disadvantages of this device are a low brightness, because only the light of a locally selectable point light source is available for illuminating the entire image, a large imaging lens and a great depth of the display, which is caused by the three-dimensional backlight. In order to confine aberration of such a large lens outside the optical axis, the focal distance must be sufficiently large, thus making the appliance very deep. Further, a three-dimensional backlight is difficult to manufacture.
A number of patents feature an additional field lens assigned to the projection system, said lens being disposed in various positions in the optical path, and thus having various functions.
Prior patent application WO2005/027534 filed by the applicant, but not published on the day of this application, also describes a multi-user display. That display contains a sweet-spot unit, a device for tracking and image control and a method for tracking the sweet-spots according to eye positions.
FIG. 1 is a top view which illustrates the working principle, but which is not to scale and does not show the full number of optical elements. A multitude of lens elements 111-114 of an imaging means 110 images switchable point illumination elements 11-46 of an illumination matrix 120 onto the observers' eyes ER, EL. Being illuminated by a large-area light source 130, the illumination matrix 120 generates at least one bundle of rays B1-B4 per lens element and observer, said bundles of rays being superimposed to form a two-dimensional sweet-spot SR at the position of the observers' eyes due to selective activation of illumination elements 11-46 by a tracking and image controller 160, imaging means 110, illumination matrix 120 and backlight source 130 form the controllable sweet-spot unit that generates a directed backlight to render an image of a transmissive LCD image display matrix 140 visible from certain spot positions in the viewing space, said positions being determined by the tracking and image controller 160. In practice, inter al. much more lens elements 111-114 and illumination elements are provided. Sub-pixels of an LCD matrix are preferably used as illumination elements.
On their way to the observer, the bundles of rays B1-B4 permeate large areas of the image display matrix 140, which alternately contains only one image of a stereoscopic image sequence of an image signal PSS. A position detector 150 determines the number of observers and their eye positions ER, EL in front of the display. The tracking and image controller 160 accordingly activates (in the example shown) the illumination elements 13, 24, 35 and 46, in order to render the current image of the stereoscopic image sequence visible from the eye position ER. As shown in FIG. 1, the illumination elements 13, 24, 35 and 46 are differently positioned in relation to the optical axes of the lens elements. If an observer moves, the tracking and image controller 160 will activate other illumination elements so to track the sweet-spot bundle according to the dislocation of the eyes. For alternate representation of the stereo images the tracking and image controller 160 renders the subsequent image visible for the respective eye of one or all observers by switching the illumination elements in synchronism with each change of the image (in the image matrix). The image is invisible for the other eye for this period of time, as that eye is at the position of a so-called dark spot. If the image sequence for the right and left eye provided by the image display matrix and the synchronised projection onto the respective eyes alternate at a sufficiently high frequency, the eyes can not distinguish the individual images presented to them. Both eyes perceive the image sequence as a stereo representation without cross-talking effects.
The bundles of rays B1-B4 practically propagate in a way that every active illumination element 13, 24, 35 and 46 is projected onto the plane of the eye positions ER or EL, enlarged to a diameter of at least several millimetres. For the sake of simplicity of the illustration of the working principle, in all figures of this document the sweet-spot is formed by parallel bundles of rays. However, in practice the propagation deviates slightly from this collimation. In any case, the sweet-spot unit is arranged so that each of the bundles of rays B1-B4 covers at least the extension of the sweet-spot area. The sweet-spot area is preferably at least as large as an observer's eye. This allows an observer to view the entire display area of the image display matrix at a homogeneous illumination and without disturbances, even if he moves a few centimetres, without the need to initiate tracking. This also greatly reduces the demands on the tracking and image controller 160 as regards its precision, function and response time.
A major advantage of the sweet-spot unit according to the above-mentioned patent application no. WO2005/027534 is that for manufacture the width of the lens elements 111-114 can be chosen independently of the pixel size in the image display matrix 140, so that each lens element covers a number N of illumination elements 11-16 at least in horizontal direction, in order to generate one sweet-spot each for N different viewing positions of each observer. For the sake of clarity the number N is six (6) in the embodiment shown. In practice, each lens element covers a greater number of illumination elements in horizontal direction. At a given cross-section of the illumination elements 11-46, the number of possible eye positions can be defined by choosing a certain cross-section of the lens elements 111-114. Therefore, it is not necessary to costly comminute the structure of the illumination matrix 130 in order to obtain a great number of possible eye positions, as it is sufficient to easily increase the width of the lens elements 111-114.
The imaging means 110 preferably consist of a lenticular or combination of several lenticulars and the lens elements 111-114 are cylindrical lenses. As a cylindrical lens generally extends over the entire height of the image display matrix 140 in vertical direction, each lens element covers several hundreds of illumination elements of the illumination matrix 120. Because the sweet-spots are only switched alternately between right and left eyes, for the sake of clarity only such elements will be considered in the description below which effect the horizontal orientation of the bundles of rays B1-B4. The illumination matrix switches over column by column. It is not relevant for the description of the present invention how many illumination elements are covered by a cylindrical lens in the vertical direction. According the present invention the tracking and image controller 160 may also track the sweet-spots in vertical direction. Analogously to horizontal tracking, the sweet-spots would be tracked by selecting respective illumination elements in the vertical direction of a two-dimensionally extended projection system.
Preferably, several adjacent illumination elements can be activated simultaneously for each bundle of rays B1-B4 in order to increase image brightness and sweet-spot width.
Another advantage of the present method is that the sweet-spot unit does not have to be modified if the image matrix is replaced.
The Technological Problem
In order to fully view the image display matrix 140, at least one illumination element per line must be activated for each lens element 111-114 and eye position. In addition, the illumination elements whose corresponding bundles of rays B1-B4 are oriented towards respective eye positions ER, EL in directions D1-D4 must be activated. As shown in FIG. 1, this is achieved in the sweet-spot unit according to patent application DE 103 39 076 by activating the illumination elements corresponding to the lens elements depending on the directions D1-D4 by the tracking and image controller 160.
In the example, an active third illumination element 13 realises the direction D1, whereas an active sixth illumination element 46 realises the direction D4. This has adverse effects on the practically usable resolution, in particular towards the margins of the illumination matrix 120, where the inventory of controllable positions is reduced. This effect considerably confines the viewing angle at which an observer can view the entire image, and thus the geometrical tracking range available for the tracking and image controller. This represents a significant disadvantage, in particular for the function of an autostereoscopic multi-user display, because a large viewing space and thus a wide range for the tracking and image controller are desirable when two or more observers are in front of the display. Moreover, even if the observer is positioned in the centre, illumination elements and lens elements are arranged at a large angle, as can be seen in FIG. 1 with the example of illumination element 46 and lens element 114. Such lenses cause aberrations, which substantially disturb the homogeneity of the projection through the image display matrix 140.
Now, this problem will be shown in more detail with the example of lens elements 111 and 114, which are situated at the margin of the imaging means 110. Whereas it is sensible to activate the illumination elements 11-13 for lens element 111, a bundle of rays B1* would be directed to a less sensible spot position S0 if the illumination element 14 were activated. Since the outermost illumination element 46 corresponding to lens element 114 is already active, lens element 114 is unable to reach spot position S0. In practice, the image display matrix 140 would therefore only be visible without the contribution of the bundle of rays B4 at position S0.
In the described example, an examination of illumination elements 15, 16, 26, 41, 42 and 43 would show similar results. Although this deficiency could be remedied for position S0 by adding an extra illumination element, which would enlarge the imaging means 110, this does not generally solve the problem, because it is not possible to add extra illumination elements corresponding to lens elements 112, 113, which are closer to the centre of the imaging means 110.
Because the distance of the illumination elements is always greater than the distance of the lens elements, large angles occur particularly at the margins of the imaging means 110, leading to significant aberrations and thus disturbing the homogeneous illumination of the image display matrix 140.